Research On Analysis For Core Saturation Stability And Leakage Magnetic Field In Large Converter Transformer

As the modern power grid voltage becomes higher and the application of EHVDC transmission,the DC bias phenomenon of large converter transformer appears to be very serious.This kind of situation will lead to core saturation instability of converter transformer,leakage magnetic field and additional loss will increase,and then the local over-heating will be caused so that the normal operation of converter transformer will be affected.The positive second harmonic component in the AC side and the fundamental component in DC side will interact with each other by AC/DC systems,and then the unstable phenomenon of core saturation of the converter transformer could be caused.Through the study of leakage magnetic field and additional loss can make the transformer as much as possible operate under the normal conditions,so that the economy and safety of converter transformer running can be improved.The value of additional loss is a very important index for the desigen and research,therefore,it is great significant to study the stability of core saturation and leakage magnetic field of converter transformer.It needs determine the complicated boundary conditions of finite element method and a lot of work need to be done,therefore,the calculation period is too longer and the optimization is not easy to achieve.But,the physical concept of analytic method is clear,the optimization program is convenient to write,and the accuracy is continually improved at the same time.Therefore,the designer hope to propose a analytic formula which can calculate additional loss of converter transformer quickly.When the converter transformer occurs the phenomenon of DC bias,the distort exciting current,the leakage magnetic field and the additional loss will be caused.First of all,the magnetic properties of electric steel sheet 27ZH100 under different DC and temperature are measured based on Epstein Frame in this thesis.The characteristics of BH curve,BP curve and permeability curve are clarified in detail,and then these curves can provide a reference for improving the accuracy of leakage magnetic field and tank loss simulation with DC bias.Then,formation mechanism of core saturation instability is studied,the interaction of positive second harmonic component in the AC side and the fundamental component in DC side is analyzed and the equivalent circuit of core saturation instability is established.The judgment factor of saturation instability is deduced by transfer matrix.The approach to determine core saturation instability is proposed.The validity of this approach can be checked on MATLAB interface.At last,the distribution of leakage magnetic field is quite complicated,it usually assumed ideal distribution in fact.That is all leakage magnetic lines parallel with axis of core all treated vertically,and then the formula of tank loss is deduced.According to electromagnetic design theory of converter transformer,the tank loss caused by leakage magnetic is calculated.The result of analytic method is coincided with the finite element method.The result of analytic method is verified as feasibility.